Scaffolded borazane-lithium hydride hydrogen storage materials

ABSTRACT

In one aspect, the invention provides a hydrogen storage composite formed of a mesoporous scaffolding material and a hydrogen storage composition comprising precursors that react to form quarternary B—H—Li—N composition. In another aspect, the invention provides a process for forming hydrogen storage material. In each aspect, a high portion of hydrogen is released as hydrogen gas and a lesser portion of hydrogen is released as a hydrogen-containing byproduct.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/625,687, filed on Nov. 5, 2004. The disclosure of the aboveapplication is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to hydrogen storage compositions andcomposite structures, the method of making such hydrogen storagecompositions and composite structures, and use thereof for storinghydrogen.

BACKGROUND OF THE INVENTION

Hydrogen is desirable as a source of energy because it reacts cleanlywith air producing water as a by-product. In order to enhance thedesirability of hydrogen as a fuel source, particularly for mobileapplications, it is desirable to increase the available energy contentper unit volume and mass of storage. Presently, this is done byconventional means such as storage under high pressure, at thousands ofpounds per square inch, cooling to a liquid state, or absorbing hydrogeninto a solid such as a metal hydride. Pressurization and liquificationrequire relatively expensive processing and storage equipment.

Storing hydrogen in a solid material provides relatively high volumetrichydrogen density and a compact storage medium. Hydrogen stored in asolid is desirable since it can be released or desorbed underappropriate temperature and pressure conditions, thereby providing acontrollable source of hydrogen.

Presently, it is desirable to maximize the hydrogen storage capacity orcontent released from the material, while minimizing the weight of thematerial to improve the gravimetric capacity. Further, many currentmaterials only absorb or desorb hydrogen at very high temperatures andpressures. Thus, it is desirable to find a hydrogen storage materialthat generates or releases hydrogen at relatively low temperatures andpressures, and which have relatively high gravimetric hydrogen storagedensity.

Therefore, in response to the desire for an improved hydrogen storagemedium, the present invention provides a method of storing and releasinghydrogen from storage materials, as well as an improved hydrogen storagematerial compositions.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a hydrogen storage mixturecomprising: (a) a hydride having one or more elements other thanhydrogen; and (b) a composition comprising X—H bonds and Y—H bonds whereX is a Group 13 element and Y is a Group 15 element. In one variation,the hydride is preferably selected from LiH, LiAlH₄ and mixturesthereof. In another variation, X is boron (B—H) and Y is nitrogen (N—H).In a still further preferred variation, the B—H, N—H composition isborazane, also named borane-ammonia complex, BH₃NH₃.

Another aspect of the present invention provides a method of storinghydrogen comprising: reacting the compositions comprising X—H, Y—H bondswith a hydride having one or more elements other than hydrogen. Thereacting forms a hydrogen storage intermediate composition comprisinghydrogen, X, Y, and at least one of the one or more elements other thanhydrogen derived from the hydride. Optionally, the X—H, Y—H compositioncomprises other elements besides X, Y and H, some of which may also beincluded in the intermediate composition.

Another preferred embodiment of the present invention provides a methodof releasing hydrogen comprising: reacting a composition comprising X—Hand Y—H bonds with a hydrogen storage hydride composition having one ormore elements other than hydrogen, wherein the reacting releaseshydrogen and forms one or more byproducts.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIGS. 1 and 2 show X-ray diffraction (XRD) patterns of the INT producthaving α, β and γ phases produced by reaction in the nLiH—BH₃NH₃ system.FIG. 1 is XRD patterns for nLiH—BH₃NH₃ (n=⅓, ½, 1 and 2). The β phasediffraction peaks are labeled with circles and the γ phase diffractionpeak is labeled with asterix. The two broad peaks in the n=⅓ sample arelabeled δ. FIG. 2 is XRD patterns for nLiH—BH₃NH₃ (n=2, 3, 4, 5 and 6).

FIG. 3 shows XRD patterns as a function of ball-milling times for then=2 composition. XRD patterns for 2LiH—BH₃NH₃ at ball-milling times ofone, two, three, and four hours. The shaded area indicates the borazanepeaks from the unreacted starting material still present in the one-hourball-milled sample.

FIG. 4 shows peak intensity of the strongest α and β peaks as a functionof temperature for the n=1 sample. XRD peak intensity versus temperaturefor the n=1 composition. The peak intensity data is taken by integratingthe area under the peak, and for the β phase the chosen peak is thedoublet at ca. 23°, while the α peak is at ca. 22.6°.

FIG. 5 shows thermogravimetric (TGA) curves for the system. TGA scansfor nLiH—BH₃NH₃ (n=½, 1 and 2).

FIG. 6 shows differential scanning calorimetry (DSC) curves as afunction of temperature in the system. DSC curves for nLiH—BH₃NH₃ (n=½,1 and 2).

FIG. 7 shows the relationship between evolved gas and temperature forn=½. Temperature dependence of the ion intensity assigned to hydrogen(×), ammonia (●), diborane (□) and borazine (+) (heating rate 5° C./min)for ½LiH—BH₃NH₃.

FIG. 8 shows the relationship between evolved gas and temperature forn=1. Temperature dependence of the ion intensity assigned to hydrogen(×), ammonia (●), diborane (□) and borazine (+) (heating rate 5° C./min)for LiH—BH₃NH₃.

FIG. 9 shows the relationship between evolved gas and temperature forn=2. Temperature dependence of the ion intensity assigned to hydrogen(×), ammonia (●), diborane (□) and borazine (+) (heating rate 5° C./min)for 2LiH—BH₃NH₃.

FIG. 10 shows X-ray diffraction for three LiAlH₄—BH₃NH₃ compositions.The open circles identify the BH₃NH₃ diffraction peaks, while the solidsquares are from aluminum.

FIG. 11 shows TGA curves for 8 mol % LiAlH₄ (triangles), 14 mol % LiAlH₄(line), 20 mol % LiAlH₄ (dots) and 30 mol % LiAlH₄ (squares).

FIG. 12 shows TGA of the 20 mol % LiAlH_(4–80) mol % BH₃NH₃ compositionball-milled for 5 min at room temperature (solid line) and at cryogenicconditions (dashed line).

FIG. 13 shows DSC curves for neat borazane (dashed), 8 mol % LiAlH₄(dash-dot-dot), 14 mol % LiAlH₄ (solid) and 20 mol % LiAlH₄ (dotted).

FIG. 14 shows the temperature dependence of the ion intensity assignedto hydrogen (H₂) (heating rate 5° C./min) for neat borazane (dashed), 8mol % LiAlH₄ (dash-dot-dot), 14 mol % LiAlH₄ (solid) and 20 mol % LiAlH₄(dotted).

FIG. 15 shows the temperature dependence of the ion intensity assignedto ammonia (NH₃) (heating rate 5° C./min) for neat borazane (dashed), 8mol % LiAlH₄ (dash-dot-dot), 14 mol % LiAlH₄ (solid) and 20 mol % LiAlH₄(dotted).

FIG. 16 shows the temperature dependence of the ion intensity assignedto a borazane byproduct (BNH_(x)) (heating rate 5° C./min) for neatborazane (dashed), 8 mol % LiAlH₄ (dash-dot-dot), 14 mol % LiAlH₄(solid) and 20 mol % LiAlH₄ (dotted).

FIG. 17 shows the temperature dependence of the ion intensity assignedto borazine ([BHNH]₃) (heating rate 5° C./min) for neat borazane(dashed), 8 mol % LiAlH₄ (dash-dot-dot), 14 mol % LiAlH₄ (solid) and 20mol % LiAlH₄ (dotted).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

In one aspect, the present invention provides a method of storing andreleasing hydrogen. In one feature, a hydrogen storage material isformed by combining precursors (a) and (b), each of which are solids.The (a) precursor is preferably a compound containing X—H and Y—H bonds,where X is a Group 13 and Y is a Group 15 element of the IUPAC PeriodicTable of Elements. Preferably X is boron (B—H) and Y is nitrogen (N—H).Most preferably, the precursor (a) is borazane. The (b) precursor ispreferably a hydride. Most preferably, the hydride is LiH or LiAlH₄.

A novel hydrogen storage composition material is formed as anintermediate (INT) in the reaction of the (a) with the (b) precursors,as described above. The formation of such an INT compound is dependentupon the individual chemical characteristics of the precursors selected,and the temperature, milling and other conditions of preparation. TheINT hydrogen storage material is preferably in a solid phase form, andis a preferred aspect in a multi-phase form. The INT hydrogen storagecomposition preferably comprises hydrogen, nitrogen, and at least one ofthe one or more elements other than hydrogen and nitrogen derived fromthe precursors. The INT hydrogen storage composition further undergoes adecomposition reaction where the stored hydrogen is released. Theproducts of this decomposition reaction are hydrogen and one or morebyproducts.

In a preferred aspect, the present invention provides a method ofstoring hydrogen in a B—H—Li—N quarternary INT hydrogen storagecomposition. The reaction between the precursors (a) and (b), forms thequarternary intermediate. Subsequent to the formation of the INT,hydrogen may be stored at suitable conditions in a stable form. When therelease of hydrogen is desired, heat and/or pressure are applied tofacilitate a decomposition reaction, where hydrogen gas is released fromthe quarternary INT hydrogen storage composition, and one or moredecomposition byproducts are formed as H₂ is released.

In another aspect, the present invention provides a method of releasingand generating hydrogen by reacting (a) composition having X—H and Y—Hbonds with a (b) hydride. The (a) and (b) precursors react to releaseand form hydrogen and one or more byproducts. In such methods of thepresent invention, the (a) and (b) precursors react to directly producehydrogen via reaction, rather than to form an intermediate (INT).Whether the INT forms is related to the thermodynamics of each reactionand the nature of the precursors.

Thus, in certain preferred embodiments, the present invention providestwo distinct physical states, one where hydrogen is “stored” and anothersubsequent to hydrogen release. Where the starting reactants reactwithout forming an INT, the hydrogenated storage state corresponds tothe precursor reactants (i.e., because a stable hydrogenatedintermediate is not formed), and the byproduct compound(s) correspond tothe dehydrogenated state.

It should be understood that in the present invention the (a) precursoris preferably a compound based on Groups 13 and 15 elements andcontaining hydrogen; more preferably is a nitride; and most preferablyis a borazane. The (b) precursor is preferably a hydride compound.Examples of such (a) and (b) precursors thus include, particularly forthe hydride, metal cations, non-metal cations such as boron, andnon-metal cations which are organic such as CH₃. Elements that formpreferred precursors of the present invention are as follows. Preferredcationic species generally comprise: aluminum (Al), arsenic (As), boron(B), barium (Ba), beryllium (Be), calcium (Ca), cadmium (Cd), cerium(Ce), cesium (Cs), copper (Cu), europium (Eu), iron (Fe), gallium (Ga),gadolinium (Gd), germanium (Ge), hafnium (Hf), mercury (Hg), indium(In), potassium (K), lanthanum (La), lithium (Li), magnesium (Mg),manganese (Mn), sodium (Na), neodymium (Nd), nickel (Ni), lead (Pb),praseodymium (Pr), rubidium (Rb), antimony (Sb), scandium (Sc), selenium(Se), silicon (Si), samarium (Sm), tin (Sn), strontium (Sr), thorium(Th), titanium (Ti), thallium (TI), tungsten (W), yttrium (Y), ytterbium(Yb), zinc (Zn), and zirconium (Zr), and organic cations including (CH₃)methyl groups.

Metal hydride compounds, as used herein, include those compounds havingone or more cations other than hydrogen, and may comprise complex metalhydrides, which include two or more distinct cations other thanhydrogen, as previously described. Examples are metal and metal alloyhydrides, such as AB₅ (LaNi₅), AB₂ (ZrMn₂), AB (TiFe), and A₂B (Mg₂Ni).Particularly preferred cations for hydrides comprise cations selectedfrom Groups 1, 2 and 13 of the IUPAC Periodic Table and particularlyfrom the group: Al, B, Ca, Li, Na, and Mg. In certain preferredembodiments, it is preferred that the cations are different species,forming the complex metal hydride, such as LiAlH₄ or LiBH₄. In certainembodiments, the metal hydride compound may have one or more cationsthat are selected from a single cationic species, such as Mg₂ and Ca₂.Preferred metal hydrides according to the present invention comprise thefollowing non-limiting examples, lithium hydride (LiH), lithium aluminumhydride (LiAlH₄), sodium borohydride (NaBH₄), lithium borohydride(LiBH₄), magnesium borohydride (Mg(BH₄)₂) and sodium aluminum hydride(NaAlH₄).

As used herein, the term “composition” refers broadly to a substancecontaining at least the preferred chemical compound complex or phases,but which may also comprise additional substances or compounds,including impurities. The term “material” also broadly refers to mattercontaining the preferred compound composition complex or phases.

According to one preferred embodiment of the present invention, thegeneral reaction for releasing hydrogen proceeds according to thefollowing exemplary mechanisms:nLiAlH₄+BH₃NH₃→H₂+byproduct;   (1)nLiH+BH₃NH₃→H₂+byproduct.   (2)The general representation is: hydride+XH—YH compound react to formhydrogen and byproduct(s). The mixture of hydride with the exemplaryborazane provides a better hydrogen storage material than borazanealone. Cold, energetic milling of the precursors provides a betterresult since the proportion of hydrogen contained in the byproduct islessened. Thus, the proportion of hydrogen released as H₂ gas isincreased.

As previously discussed, in certain preferred embodiments anintermediate hydrogen storage composition is formed, which is expressedby the following general reaction:nLiH+BH₃NH₃→hydrogen+intermediate,where the INT is a new B—H—Li—N quarternary system havingpreviously-unknown phases, α, β and γ. Here, a 2:1 LiH+BH₃NH₃ formsnominally, Li₂BNH₈, containing the α, β and γ phases and LiH.

Although not wishing to be limited to any particular theory, a novelsolid quarternary intermediate compound is known to occur where thehydride has one or more M′ cations selected as Li, and generallybelieved to occur where M′ is selected from Groups 1 and 2 of the IUPACPeriodic Table and particularly the group consisting of: Li, Ca, Na, Mg,K, Be, and mixtures thereof, and where X—H, Y—H precursor is anitrogen-hydrogen precursor comprising a Group 13 element from the IUPACPeriodic Table. The preferred precursor is borazane. Where the novel INThydrogen storage composition is formed, such a composition undergoes adecomposition reaction mechanism, to form a dehydrogenated state whereone or more decomposition byproducts are formed as hydrogen is released.

Other non-limiting examples of alternate preferred embodiments accordingto the present invention where hydrogen generation occurs, include thefollowing exemplary precursors and systems. LiH is substituted with NaH,KH, MgH₂, and/or CaH₂. Examples are the NaH—BH₃NH₃ and MgH₂—BH₃NH₃systems. LiAlH₄ is substituted with NaAlH₄, LiBH₄, NaBH₄, LiGaH₄ and/orNaGaH₄. Further, BH₃NH₃ is substituted with BH₃PH₃, AlH₃NH₃, and/orAlH₃PH₃.

Preferred conditions for reactions of the invention vary with respect topreferred temperature and pressure conditions for each independentreaction. However, it is preferred that the reaction is carried out as asolid state reaction, in a non-oxidizing atmosphere, essentially in theabsence of oxygen, preferably in an inert atmosphere, such as undernitrogen or argon. Further, as will be discussed in more detail below,it is preferred that the solid precursors are reduced in particle sizefrom their starting size and/or energetically milled.

After the novel INT hydrogen storage composition is formed, it is ahydrogenated and stable material. When release of the hydrogen isdesired, the composition is heated and hydrogen release preferablyoccurs at a temperature of between about 80° C. and 170° C. at ambientpressure.

EXAMPLE 1

This example is of the nLiH—BH₃NH₃ system. Samples used produced withhigh-energy ball-milling of combinations of LiH and BH₃NH₃. The systemwas studied using X-ray diffraction, thermal analysis and massspectrometry. Three quarternary phases, designated α, β and γ, werediscovered. Of these three phases, at least two, the α and β phases,exhibit hydrogen storage properties. The a phase releases ca. 10 wt %hydrogen below 150° C. This hydrogen release is slow, taking place overa ca. 20–30° C. The β phase has not been obtained in pure samples,analysis suggests approximately 25–40% of the sample is the β phase.This phase mixture has a 3 wt % hydrogen release at ca. 80° C. In bothdecomposition reactions, ammonia was seen as a byproduct in smallquantities. For the α phase, diborane and borazine were additionalbyproducts. Furthermore, the hydrogen release is exothermic for bothreactions. Rehydrogenation is in process.

Both LiH and BH₃NH₃ were purchased from Aldrich, with nominal technicalpurities of 99% and 90+%, respectively. For borazane, the majorimpurities were residual solvents.

Ball-milling was carried out in a Spex 8000 mixer mill using two 1.27 cmdiameter and four 0.635 cm diameter hardened steel balls of aggregatemass 21 g for a 2 g typical sample mass. The ball-milling vessels wereO-ring sealed hardened steel jars under 1 atmosphere of Ar. The millingtimes were varied between 30 minutes and 12 hours to ensure completereaction of the starting materials. In the n≧2 samples, ball-milling atleast two hours was required for complete reaction between the LiH andBH₃NH₃. However, for lower LiH concentrations ball-milling times of onehour was sufficient to obtain equilibrium.

X-ray diffraction (XRD) was performed using a Siemens D5000diffractometer and Cu Kα radiation. Diffraction patterns were collectedbetween 5 and 85° 2θ at 0.020° increments. Samples were loaded underargon, and protected using a thin XRD-transparent film. Data analysiswas performed using the Bruker EVA software package. The real-timein-situ XRD experiments were carried out in closed XRD capillary tubewith a Bruker AXS General Area Detector Diffractometer System (GADDS)using Cu Kα radiation. The capillary tubes were filled and sealed underargon. Diffraction patterns were collected every minute while heatingthe sample at 1° C./min and recording the pressure.

The combined thermogravimetric (TGA), differential scanning calorimetry(DSC) and mass spectrometry (MS) technique was used to analyze thegaseous component while monitoring the weight loss and heat flow of thesample. The instrument used for this combined study was a Netzsch STA409 unit equipped with a quadrupole mass spectrometer Pfeiffer QMG422via a two-stage pressure reduction system with an alumina skimmer. Thisequipment allows for the detection of unstable products immediatelyafter their formation (within subseconds). The system was evacuated andflooded with high-purity argon. The measurements were performed underargon (30 mL/min) in the dynamic mode. Sample sizes ranged from 5 to 20mg, and the heating rates used were 5° C./min and 1° C./min from 25 to250° C. The slower heating rate was used for the n<2 composition samplesto avoid excessive foaming during analysis. Both TGA/DSC and signalsfrom the mass spectrometer in the SIM (selective ion monitoring) modewere recorded.

RESULTS OF EXAMPLE 1

X-Ray Diffraction (XRD)

XRD patterns of the different compositions are shown in FIGS. 1 and 2.All the samples have been ball-milled until BH₃NH₃ was no longerdetected. For LiH-poor samples (n=⅓, ½, and 1) one hour was sufficient,while the LiH-rich samples required two hours of ball-milling. Forclarity the LiH peak positions have been marked. In this system, theobserved phases vary with LiH content. At small n-values, thepredominant phase is assigned the α-phase, which is seen nearly as apure phase in the n=½ sample. At this composition, only a very smallamount of LiH is detected in addition to the α-phase. In the n=⅓ sample,some broader peaks are observed which are not consistent with theα-phase. These broad peaks have been labeled δ. LiH is not detected inthe n=⅓ composition. With increasing LiH content, new phases appear inthe diffractograms. The first new phase is seen for n=1, where at ca.23–124° new peaks are seen. The new diffraction peaks are labeled withcircles in FIG. 1. For n=2, a third phase is present, assigned γ-phase,giving a total of four phases, LiH, α, β and γ. The γ-peak at ca. 22° islabeled with asterisks in FIG. 1. There are many weaker reflections thatappear with increasing LiH content, both for n=1 and n=2. The weakerreflections are considered resolvable to a respectivepreviously-determined phase. The peak at ca. 26° appears first for n=1as a strong middle peak in a triplet. Increasing the LiH to the n=2composition causes the intensity of the 26° peak to increase further,while the two β peaks at 23° lose intensity (FIG. 1). Further increasingthe LiH content, to n=5 (FIG. 2) results in a doublet in the 26° area,instead of the triplet that was there for n=1. This indicates that the βand γ phases have overlapping reflections in this region. Therefore, asa positive identification of the three phases α, β and γ, one singlestrong feature has been chosen; for α the strong peak at 22.6°, for βthe doublet at 23° and for γ the peak at 21.4°.

One feature that should be noted is that the increasing LiH content inthe samples causes the α and β phases to become amorphous. FIG. 2 showsthat for the n=4 composition only very broad features are left of the αpeak at 22.6° and the β doublet at 23°. The γ peak at 21.4°, however, isstill strong and relatively sharp. At n=5, only the γ signature peak isleft, together with two more broad features and the LiH peak, and atn=6, only the LiH peak is still present. It should be noted that eventhough the α, β and γ peaks show significant broadening of the XRD peaksupon increased LiH content, the LiH peak itself does not appear tochange significantly.

FIG. 3 shows the XRD patterns as a function of ball-milling time for then=2 composition. There is a loss of crystallinity with increasingball-milling time. The only phase that appears unchanged by theball-milling is LiH. When the ball-milling exceeds two hours, thediffraction peaks for the α and β phases broaden and lose intensity.After four hours, little is left of the α and β peaks leaving just the γphase and LiH. However, the γ phase is also substantially broadened bythe prolonged ball-milling. The loss in crystallinity with increasedball-milling time is typical for samples with larger LiH content (thisit true also for the n=½ and n=1 compositions). First, the α and βphases become amorphous, then the γ phase becomes amorphous at a laterstage while LiH remains unchanged.

In order to asses the high temperature stability of the different phasesin this system, in-situ XRD experiments were performed on the n=½, 1 and2 samples. These three compositions represent all the phases observed inthe quasi-binary LiH—BH₃NH₃ system. FIG. 4 shows the peak intensity ofthe strongest α and β peaks as a function of temperature for the n=1sample. The pressure in the capillary tube is also plotted. The β phase(crosses) clearly decomposes much earlier than the α phase (squares).The β peaks have completely disappeared at 80° C., while the α peaks arepresent in the sample until 130° C. The pressure curve indicates atwo-step pressure increase suggesting that both phases decompose byreleasing a gas. The γ phase was not present in the in situ XRD datashown in FIG. 4, so a three-hour ball-milled n=2 sample was alsostudied. The γ peak intensities disappear at the same time as those ofthe β phase. Based on this, and the fact that some of the peaks overlap,it is difficult to separate the β and γ phase decompositions, anddetermine each phase contribution to hydrogen storage at lowtemperatures.

Because the α-phase was obtained as a nearly pure phase, a crystalstructure could be determined. Table 1 displays the spacings,intensities and assigned indices. The indices relate to a tetragonalcell with a=4.032 angstroms and c=17.001 angstroms. From the systematicabsences, the α-phase can be assigned the space group P-42₁c. Thelattice parameters give a cell volume of 276.41 cubic angstroms, nearlytwice the cell volume (139.72 cubic angstroms and 134.65 cubicangstroms) for the two crystal structures of BH₃NH₃ reported in theJCPDF. Therefore, it would be expected that the α-phase would have fourBH₃NH₃ molecules per unit cell, twice that of neat BH₃NH₃, and two Liatom per unit cell. Assuming no hydrogen is lost during the formation,the calculated density of the α-phase would be 0.837 g/cm³. Though thecell volume of the α-phase is twice that of BH₃NH₃, neither of thetetragonal lattice parameters have a rational relationship to thosereported for BH₃NH₃. Therefore, XRD data indicates that the BH₃NH₃molecules have a significantly different arrangement in the α-phase thanBH₃NH₃, yet the cell volumes of the different structures is determinedonly by the number of BH₃NH₃ molecules.

Thermal Analysis

TGA curves for nLiH—BH₃NH₃ (n=½, 1 and 2) samples are shown in FIG. 5.The n=½ and 1 samples have been ball-milled for one hour and the n=2sample for two hours, yielding samples with no residual BH₃NH₃. Thesethree samples have been chosen since they are representative of thephases observed in the LiH—BH₃NH₃ system. The n=1 and 2 samples exhibita two-step weight-loss process. The first step is initiated at ca.70–80° C., and is fairly rapid. This is also the most pronounced step.The second step is quite slow, and starts around 120° C. and typicallycontinues for 20–40° C. Above 180° C., the decomposition reaction hascompleted. For the n=½ sample, the weight loss appears to be asingle-step process. The single step corresponds to the second, slowerstep seen in n=1 and 2. However, there is a small, gradual weight lossstarting all ready at ca 80° C. indicating some small amount ofdecomposition is taking place at low temperatures. The n=⅓ compositionbehaves practically identical to the n=½ composition. For higher LiHcontent (n=3, 4, 5 and 6), no new features are seen and the only changeis in the total weight loss of the sample. The total weight loss becomessmaller, since the excess LiH in the sample acts as a filler at thesetemperatures.

It can be seen from FIG. 5 that the weight loss decreases as the LiHcontent increases. The n=½ sample has the largest overall weight loss,at ca. 10 wt %. Increasing the LiH content to n=1 gives a weight loss ofca 7 wt. %, and n=2 has an even smaller weight loss, at ca. 5 wt. %. Themaximum theoretical weight loss for the samples can be estimatedassuming complete loss of hydrogen. If all hydrogen is released, thetheoretical weight loss would be ca. 18 wt % for the n=1 sample, asillustrated in Equation 1. The complete dehydrogenation of thiscomposition:BH₃NH₃+LiH→LiBN+3.5 H₂; 18 wt. % H₂   Equation 1However, XRD patterns show that some LiH is remains in the samples afterball-milling, and LiH releases its hydrogen above 550° C. Due to thehigh temperature of the decomposition of LiH, the n>1 samples would ofcourse have smaller, rather than larger, theoretical weight losses,since the excess LiH does not contribute to the hydrogen storage at thecurrent temperature range up to 200° C.

The thermal behavior as a function of ball-milling time has also beenstudied for all sample compositions. Longer ball-milling times result ina smaller weight loss for most compositions. When samples areball-milled for tow hours or longer, there is a substantial pressurebuild up inside the ball-milling vessel. This excess pressure is due tothe partial decomposition of the material during ball-milling.Therefore, a smaller weight loss seen in the TGA could mean that part ofthe material was already decomposed during ball-milling. When the sampleis ball-milled overnight (12 hours), no weight loss was observed at all.This sample has a completely amorphous XRD pattern.

Differential scanning calorimetry has been used to study the heat flowduring the decomposition reaction. FIG. 6 shows the DSC curves as afunction of the temperature for the n=½, 1 and 2 compositions. The n=½composition shows a sharp well defined exothermic signal thatcorresponds to the weight loss at ca. 120° C. This exothermic peak isfollowed by a second small exothermic feature at ca. 140°. A smallpossible endothermic feature is present at ca. 160° C., and is directlyfollowed by a third small exothermic peak. These last three events arerather small compared with the first feature, and when compared to FIG.5, they do not seem to correspond to a weight loss. The endothermic peakcould be a partial melting of the sample. A complete melting has notoccurred as seen by the still powdery nature of the sample after theexperiment is complete. Due to the upward sloping background in the DSCcurves caused by the weight loss in the sample, the amount of overlapbetween the small exothermic and endothermic features is difficult todetermine.

The n=1 composition has a rather broad exothermic signature in thetemperature range of the first weight-loss step at ca. 70–80° C. Asecond exothermic feature is seen for the second weight loss at ca. 140°C. This second feature is much smaller than the first, but sharper.There are no apparent extra endo- or exothermic peaks present for thiscomposition. In the case of the n=2, and more LiH rich samples, they alldisplay a small endothermic peak before the first weight loss occurs, anendothermic peak that becomes overshadowed by the exothermic nature ofthe weight-loss reaction. This endothermic event might be a meltingreaction that starts before the decomposition and is prevented frombeing completed by the decomposition reaction. The first step of theweight loss is associated with a single strong exothermic peak. Thesecond weight loss corresponds with two very weak and broad exothermicsignals. Based on the DSC data for all the different samplecompositions, it is seen that all three phases, α, β and γ, decomposeexothermically.

In order to determine which gases evolve during the decomposition of thesamples, mass spectrometry was used. The MS data was collectedsimultaneously with TGA and DSC data, to be able to fully correlate theevolved gases with each step in the decomposition process. For the n=½composition, a single-step weight loss is seen. Hydrogen is beingreleased during this weight-loss step (FIG. 7). A small dip in thehydrogen signal is seen at ca. 140° C. Maximum in the hydrogen ioncurrent signal is at close to 200° C. Hydrogen is a very light gas, andit takes time for it to move from the sample into the mass spectrometerchamber, and also to move back out. Therefore, the signal does notreturn to the baseline even after H₂ no longer is being released. NH₃ isalso present in the gas phase. This NH₃ is released at about the sametemperature as the H₂, thereby making it difficult to avoid ammoniathrough changing the heating parameters. Other gaseous species arepresent in very small quantities, and include B₂H₆, BNH and (BHNH)₃(FIG. 7). Ref. 1 gives a comprehensive account of the decomposition ofpure BH₃NH₃ and the resulting evolved gaseous species.

A two-step decomposition is seen for n=1, and there is a small amount ofhydrogen coming off in the first step, at ca. 60–70° C. (FIG. 8). Thereis also a substantial amount of ammonia being released at thistemperature. The main amount of hydrogen is released in the second step,with a maximum at ca. 150° C. Very small amounts of diborane, and otherBNH decomposition products are released mostly in the second step. Then=2 sample has the same general behavior as the n=1 composition, butwith a smaller overall weight loss. The difference in the two steps ofthe gas release is more difficult to see for this composition, with theearly hydrogen release being overshadowed by the strong broad maximum inthe ion current for hydrogen at ca 150° C. (FIG. 9). Increasing theamount of LiH further only causes a smaller overall weight loss, anddoes not influence the relative amounts of each evolved gas species. Thesame is true for n=⅓, which shows the same thermal and mass spectrometrydata as the n=½ sample.

These new phases are surprising since the expectation was to substituteLi for H on the borazane molecule. However, here new crystallographicphases have been discovered. In fact, one of the phases discovered inthis study (the α phase) is a borazane dimer, where two borazanemolecules are linked together through a lithium bridge. There are atotal of three new phases found in this system. All of them store largeamounts of hydrogen. The phases found in this study show excellenthydrogen storage capabilities. A maximum of 10% weight loss below ca.150° C. can be obtained for the n=½ composition. This is a substantiallylarger hydrogen storage capacity compared with other classes ofmaterials.

TABLE 1 XRD d-spacings data for the α-phase. Peaks positions wereobtained from the ½LiH—BH₃NH₃ sample. Relative (hkl) d_(o) d_(c)Intensity (002) 8.5319 8.5005 589 (004) 4.2582 4.2503 88 (101) 3.92763.9234 4426 (102) 3.6445 3.6431 129 (103) 3.2806 3.2854 102 (104) 2.92732.9252 59 (110) 2.8527 2.8512 790 (112) 2.7043 2.7032 183 (114) 2.36852.3678 122 (008) 2.1249 2.1251 32 (107) 2.0797 2.0805 37 (200) 2.01572.0161 81 (201) 2.0026 2.0021 41 (210) 1.8034 1.8033 34 (211) 1.79311.7932 77 (213) 1.7188 1.7184 22 (109) 1.7100 1.7106 30 (118) 1.70251.7039 15 (1011)  1.4436 1.4432 28 (220) 1.4266 1.4256 29 (0012)  1.41591.4168 41 (1112)  1.2696 1.2688 40 (1013)  1.2430 1.2440 65

The n=½ composition has the best hydrogen storage capacity of thesamples tested. The samples were not single phase. High temperature XRDcombined with TGA, DSC and MS data indicate that the α phase releaseshydrogen at ca. 150° C., while the β and γ phases has a lowerdecomposition temperature of ca. 80°. The decomposition behavior of then=1 sample, containing both α and β, therefore reflects the behavior ofboth these phases. In fact, the rapid first decomposition step can beattributed to decomposition of the β phase, while the slower second stepcan be attributed to the α phase. Since the n=1 sample contains mostly αand β, with some small addition of LiH (ca 10–20%), and based on thefact that the weight loss from the α phase is 40% of the full weightloss seen for the n=½ sample, there is about 40% α in the n=1 sample.Thus, there is also about 40% β in this sample. Based on this, thehydrogen storage capacity of the β phase is estimated at 6–7%. This is asubstantial weight loss, and indicates that the β phase is attractivefor hydrogen storage purposes. When viewing the DSC data, it seems thatthe exothermic signal for β decomposition is quite weak, signalingdecomposition energetics favorable for recycling purposes. The MS datadoes suggest that some NH₃ is evolved in addition to the hydrogen. Basedon this, a decomposition reaction is thought to be:β→qH₂ +rNH₃+amorphous white solid; 80° C.α decomposes in much the same way as β, releasing hydrogen and alsosmall amounts of NH₃, diborane and borazine. Based on this, adecomposition reaction is thought to be:α→qH₂ +rNH₃ +zB₂H₆+amorphous white solid; 150° C.The high temperature XRD and TGA data suggest that γ decomposes at thesame temperature as the β phase. Since the γ phase is seen together withthe β phase in most samples, it is not possible to distinguish which ofthe decomposition products can be assigned to γ alone. Therefore, thesame decomposition reaction is proposed for γ as for β:γ→qH₂ +rNH₃+amorphous white solid; <80° C.

The apparent rate of decomposition for the β and γ phases, combined withthe low decomposition temperature makes β and γ attractive for hydrogenstorage. The hydrogen release is an exothermic process so thatrehydrogenation is an economic challenge.

The results of the decomposition of α, β and γ have some majordifferences compared with the hydrogen storage properties of borazaneitself. Pure borazane has been show to have a 14 wt % mass loss, butwith additional decomposition products being NH₃, B₂H₆, (BHNH)₃ andothers. The hydrogen is released in a two-step process, where borazanedecomposed first to BH₂NH₂, and subsequently to polymeric BHNH. Incontrast, the phases in the present invention decompose throughsingle-step processes.

In summary, the nLiH—BH₃NH₃ (n=⅓–6) system has been demonstrated hereand studied using X-ray diffraction (XRD), thermogravimetric analysis(TGA), differential scanning calorimetry (DSC), mass spectrometry (MS)and a combined DSC/TGA/MS technique. This system contains severaldifferent phases, some of which release hydrogen below 150° C., withadvantages as described above.

Accordingly, based on the above example, it is apparent that there arethree new, previously unknown, phases, designated α, β and γ, in thenLiH—BH₃NH₃ system. The α phase can store up to 10 wt % hydrogen, andrelease this hydrogen below 150° C. The hydrogen release is anexothermic event. From the XRD data, the α phase has been identified asa primitive tetragonal crystal structure with a P-42₁c space group andlattice parameters of a=4.032 angstroms and c=17.001 angstroms. Thehydrogen release for the β phase is very fast. The β phase releaseshydrogen at ca. 80° C. Due to impurities in the samples containing the βphase, an experimentally-based estimate of the hydrogen storage capacityfor the β phase was determined. It is estimated to be between 6 and 12wt % based on the amount of impurities present. The B—H—Li—N quarternarysystem contains many new useable hydrogen storage phases.

EXAMPLE 2

This example is for new hydrogen storage materials, as well as thepreparation method. The materials are mixtures of borazane (BH₃NH₃) andlithium alanate (LiAlH₄).

Cold milling borazane and LiAlH₄ produces hydrogen storage materialswith thermal properties that differ from those of starting materials.LiAlH₄ additions to borazane reduce the exothermicity of desorption andthe amount of byproducts. An optimum concentration of 20 mol % LiAlH₄was observed. An exemplary preparation method follows.

The mixing (ball-milling) was done under quasi-cryogenic conditions atpreferred temperature of liquid nitrogen, about −195° C. Temperaturesbelow room temperature are usable, with temperatures between about 25°C. and −195° C. The combination of mixing borazane with lithium alanateand milling it below room temperature resulted in a hydrogen storagematerial that had less byproducts than pure borazane and one thatdesorbed hydrogen with less exothermicity than pure borazane. Thecalorimetry is important because endothermic desorptions and very weaklyexothermic desorptions are thought to be reversible.

The raw materials were first milled separately in an inert atmosphere(argon) at room temperature for 30 minutes to reduce the particle size.Next, 0.5 g of the milled borazane and the appropriate amount of milledlithium alanate were milled together, again in an inert atmosphere. Eachvessel was retrofitted with a steel cap to promote a good seal with thevessel at cryogenic conditions.

Cold milling was accomplished by first dipping the vessel in a bath ofliquid nitrogen for approximately two hours to cool, before ball-millingstarted. Mixing was allowed to occur for no more than five minutes dueto sample heating once out of the liquid nitrogen bath. If more millingtime was desired, the vessel was first placed in liquid nitrogen forfifteen minutes to cool. After all of the milling was completed, thevessel was again inserted into the liquid nitrogen bath forapproximately one hour before it was allowed to warm up to roomtemperature overnight.

RESULTS OF EXAMPLE 2

FIG. 10 shows the X-ray patterns for the xLiAlH₄(100-x)BH₃NH₃ startingcompositions (x=8, 14 and 20). All samples have been ball-milled undercryogenic conditions for 5 min. For the x=8 composition, the maindiffraction peaks can be ascribed to BH₃NH₃ (open circles) with onlyvery small amounts of Al-metal (solid squares) as a ball-millingproduct. There is no evidence for any LiAlH₄ starting material stillbeing present in the sample. It is therefore assumed that all lithiumalanate is consumed during ball-milling, and the Al-metal is formed as aresult of this reaction. Increasing the lithium alanate content in thesample increases the intensity of the aluminum peaks, and decreases theintensity of the borazane peaks. Further increase of LiAlH₄ results in arapid decrease in the borazane peak intensity without a correspondinglystrong increase in the aluminum peak intensity (x=20 composition). Thereis one very broad feature at ca 10–15° which is strongest for the x=14composition. This feature is not consistent with either of the startingmaterials, and also not consistent with aluminum metal. From the X-raydiffraction data, it seems that there is a chemical reaction between thestarting components during the cryogenic ball-milling, and the productof this reaction does not have any crystalline X-ray diffractionpattern. The amorphous nature of this reaction product is probablypartly due to the cryogenic conditions under which it was formed, thelow temperature slowing down diffusion in the sample. However, the samestarting composition and ball-milling times at room temperature does notproduce any crystalline products either. Rather ball-milling thesematerials together at room temperature results in decomposition of thematerial inside the ball-milling vessel. The only crystalline productfound in the diffraction pattern is aluminum metal, and there is asubstantial overpressure inside the vessel, stemming from gaseousdecomposition products. It is quite curious that there are nocrystalline lithium phases to be found, and the nature of the lithium inthe reaction products is not known.

FIG. 11 shows TGA curves for different xLiAlH₄(100-x)BH₃NH₃ compositions(x=8, 14, 20, and 30). All samples have been ball-milled under cryogenicconditions for 5 min. The x=8 composition has a two-step weight loss,where the two steps are largely overlapping. The first weight lossstarts at ca. 100° C., followed by a new, larger, weight loss at ca.120° C. This two-step reaction is quite similar to that seen by pureborazane. The first decomposition step for BH₃NH₃ is at ca. 100° C., andthe second at ca. 130° C. It is not surprising, given the large excessof BH₃NH₃ in the x=8 sample, that borazane decomposition dominates theoverall weight loss picture. For the other compositions, only a singleweight loss is observed, with an onset of ca. 100° C. for allcompositions. This means that these more alanate-rich compositionsbehave differently than borazane, indicating again that the ball-millinghas produced new materials.

A comparison between the weight loss after cryogenic and roomtemperature ball-milling is seen in FIG. 12, where the TGA curves forx=20 at room temperature (solid line) and cryogenic (dashed line)conditions is shown. As seen, there are more gaseous byproducts beingformed during the decomposition of the room temperature sample comparedwith the cryogenically ball-milled sample. Both samples have beenball-milled for 5 min. The trend is apparent for other compositions andball-milling times as well, and it is clear that less byproducts form ifthe sample has been ball-milled under cryogenic conditions.

FIG. 13 shows the DSC signals in the range 50–200° C. where most of thethermal events occur. It is seen that the DSC signal is affected to alarge extent by the LiAlH₄ concentration. When heated, borazane aloneusually shows a small endothermic melting followed by a strongexothermic event that occurs at ca. 100° C. (the same temperature as amajority of the mass loss). The addition of LiAlH₄ reduces theexothermic behavior of the material significantly. The small endothermicmelting feature also disappears with increasing LiAlH₄ content, and infact, by adding 20 mol % LiAlH₄ the resulting sample is almostthermo-neutral. However, by adding even more LiAlH₄, 45 mol %, thematerial that is produced after milling does not have any measurableweight loss nor does it have a thermal event in the temperature range weare interested in for automotive applications.

The amounts of hydrogen and byproducts (i.e. NH₃ and (BHNH)₃) thatdesorb from each of the samples are shown in FIGS. 14–17. Since the massspectrometer was not calibrated, the results are not presented inconcentrations but rather as ion currents in units of current per massof sample. It is seen from FIG. 14 that hydrogen is the major componentof the gas phase during decomposition, and that its detectioncorresponds very well to the weight loss registered by the TGA. However,both NH₃ and (BHNH)₃ are also seen at the start of the weight loss, thusmaking it difficult to construct a temperature scheme that willcompletely eliminate the contamination. What is seen, however, is thatthe amount of LiAlH₄ present in the sample greatly reduces the amount ofcontaminants. Table 2 shows the area under each of the mass spectrometersignals for each product normalized to the amount of borazane in thesample. This normalization was done to confirm that the alanate was notjust a dilutant. The 20 mol % alanate system cold milled for 5 minreduces the amount of NH₃, BNH_(x) and (BHNH)₃ that desorbs by more thanany of the other samples. As compared to borazane alone, the NH₃ andBNH_(x) concentrations are reduced by an order of magnitude, while the(BHNH)₃ concentration is reduced by nearly an order of magnitude. Itappears from this set of experiments that there is an optimumconcentration of 20 mol % LiAlH₄ and milling for more than 5 minutes isnot beneficial because more byproducts desorb upon heating.

In summary, the cold milling, or cryogenic milling, process of thepresent invention is conducted at a temperature below ambient or belowabout room temperature, nominally considered to be 25° C., desirably ata cold temperature less than −100° C., and preferably and convenientlyat the temperature of liquid nitrogen of about −195° C. In this coldcondition, metastable phases are achieved that would otherwise not formduring room temperature ball-milling. Regular room temperatureball-milling essentially drives the system to a completelydehydrogenated state, and the resulting product is not capable ofaccepting any hydrogen. Cooling down the materials before ball-millingand subsequent cold milling conditions creates an environment wherecompounds that are unstable at room temperature can exist. The kineticsare slow enough, even at ambient conditions, that the decomposition ofthese products can be controlled by gentle heating. Accordingly,reaction is conveniently controllable so that ambient conditions providesustained shelf life. Thus, the product has sustained shelf life atambient conditions and further extended shelf life at cooler conditions.

The present invention overcomes difficulties encountered when precursormaterials are mixed together at essentially ambient conditions and wheresuch mixing generates heat due to the energy of impact, resulting indehydrogenated hydrogen storage materials that substantially evolvebyproducts with no hydrogen or very low portion of hydrogen released inthe form of hydrogen gas.

Therefore, the present invention achieves mixing at a temperature thatdoes not initiate a hydrogen release reaction and that facilitatesrelease of hydrogen under conditions where a higher proportion ofsubstantially pure hydrogen gas is released out of the materials, and alesser proportion of hydrogen is bound in undesirable decompositionproducts. The process also improves thermal behavior of the materialslending itself to a thermodynamic system for regeneration orrehydrogenation. Therefore, the system and method provides hydrogenrelease with a thermic control, as compared to conventional systems andmethods.

Conveniently, when it is desired to release hydrogen from the system,the system may be permitted to simply heat to room temperature; however,the release of hydrogen is very slow and even at room temperature, thehydrogen storage material has an essentially stable shelf life, at leastfor a period of months, due to the very slow decay time. Conveniently,when it is desired to evolve hydrogen at a high-volume rate, the systemis heated to about 100° C. to evolve hydrogen. Such evolution occurswith a lower proportion of undesirable byproduct compounds, as describedearlier.

Advantageously, according to the above, the invention provides a newmaterial that is essentially stable at room temperature in a suitableatmosphere or environment and at least meta-stable at room temperature.Such suitable atmosphere or environment constitutes one which does notreact with the material, is essentially inert with respect to thematerial and is desirably non-oxidizing, and preferably a vacuum orinert atmosphere. Representative inert gases are argon and helium andthe like. The material formed by the process of the invention iskinetically inhibited from decomposition until heat is added, preferablyto 100° C.

The preferred combination of precursors is about 20 atomic percentlithium alanate (LiAlH₄) and about 80 atomic % of the borazane. The coldball-milling process is conveniently conducted for about five minutes,and the weight percent evolution is about 16%, which is very attractiverelative to the theoretical maximum of hydrogen evolution on the orderof 17–18%.

TABLE 2 Effect of LiAlH₄ concentration on Desorption Product QuantitiesLength of Cold LiAlH₄ Milling H₂ NH₃ BHNH B₃N₃H₆ Conc. [mol %] Time[min] [A s mg⁻¹] [A s mg⁻¹] [A s mg⁻¹] [A s mg⁻¹] 8 5 1.28 × 10⁻⁷ 3.35 ×10⁻⁸ 2.12 × 10⁻⁸  9.3 × 10⁻⁹ 14 5 3.74 × 10⁻⁷ 5.26 × 10⁻⁹ 1.98 × 10⁻⁹6.42 × 10⁻⁹ 20 5  1.6 × 10⁻⁷ 2.24 × 10⁻⁹ 1.28 × 10⁻⁹ 2.77 × 10⁻⁹ 14 10 4.36 × 10⁻⁷ 1.54 × 10⁻⁸  9.42 × 10⁻¹⁰ 9.78 × 10⁻⁹ 0 — 6.75 × 10⁻⁷ 1.45 ×10⁻⁸ 3.48 × 10⁻⁸ 1.38 × 10⁻⁸

All of the aforementioned hydrogen storage materials, as produced by theaforementioned processes, are preferably deposited on porous scaffoldingmaterial to form a composite hydrogen storage material. The scaffoldingmaterials can be silica based materials, metal-organic frameworkmaterials, zeolite type materials, an alumina-based material, orcarbon-based porous material. It is envisioned the scaffolding materialhas an average pore diameter of 1–5 nm and, preferably, an average porediameter of 2–4 nm. The scaffolding material additionally has a surfacearea which is greater than about 450 m²/g and, preferably, greater thanabout 500 m²/g.

The scaffolding material is optionally coated with AB2, AB5, AB, A2Btype materials, Sodium Alanate alone, or LiNH₂+LiH, LiNH₂+LiBH₄, ormixtures thereof. The scaffolding material is preferably coated with aLiBNH or LiBNAlH hydrogen storage material, and most preferably thefamily of LiAlH₄—BH₃NH₃ or LiH—BH₃NH₃ materials described above.

The hydrogen storage materials are preferably dissolved in a non-aqueoussolution and deposited onto the mesoporous scaffolding material. In thisregard, the non-aqueous solution is preferably a cyclic ether such astetro hydro furan, which is driven off the mesoporous-scaffold to form ahydrogen storage composite structure.

The resulting composite structures are rechargeable hydrogen storagematerials which have a reduced heat of decomposition when compared totheir non-composite hydrogen storage analogs. Further, the compositestructures are formed in such a manner that only hydrogen is evolved anddetected outside of the scaffolding during decomposition.

In one embodiment of the invention, a composite hydrogen storagematerial is formed having a mesoporous scaffolding material with amedian pore size of about 2–4 nm, and a surface area of greater than 500m²/g. The mesoporous scaffolding material is coated with hydrogenstorage material such as LiB₂N₂H₁₃, LiBNH₇, Li₂BNH₈, 0.2 mol LiAlH₄–0.8mol BH₃NH₃ mixture and mixtures thereof.

In another embodiment of the invention, a composite hydrogen storagematerial is formed having a silica-based mesoporous scaffolding materialwith a median pore size of about 2–4 nm, and a surface of greater than500 m²/g. The silica-based mesoporous scaffolding material is coatedwith hydrogen storage material such as LiB₂N₂H₁₃, LiBNH₇, Li₂BNH₈, 0.2mol LiAlH₄–0.8 mol BH₃NH₃ mixture and mixtures thereof.

In another embodiment of the invention, a composite hydrogen storagematerial is formed having a carbon-based mesoporous scaffolding materialwith a median pore size of about 2–4 nm, and a surface of greater than500 m²/g. The carbon-based mesoporous scaffolding material is coatedwith hydrogen storage material such as LiB₂N₂H₁₃, LiBNH₇, Li₂BNH₈, 0.2mol LiAlH₄–0.8 mol BH₃NH₃ mixture and mixtures thereof.

In another embodiment of the invention, a composite hydrogen storagematerial is formed having an alumina-based mesoporous scaffoldingmaterial with a median pore size of about 2–4 nm, and a surface ofgreater than 500 m²/g. The aluminum-based mesoporous scaffoldingmaterial is coated with hydrogen storage material such as LiB₂N₂H₁₃,LiBNH₇, Li₂BNH₈, 0.2 mol LiAlH₄–0.8 mol BH₃NH₃ mixture and mixturesthereof.

In another embodiment of the invention, a composite hydrogen storagematerial is formed having a mesoporous zeolite scaffolding material witha median pore size of about 2–4 nm, and a surface of greater than about500 m²/g. The zeolite scaffolding material is coated with hydrogenstorage material such as LiB₂N₂H₁₃, LiBNH₇, Li₂BNH₈, 0.2 mol LiAlH₄–0.8mol BH₃NH₃ mixture and mixtures thereof.

In another embodiment of the invention, a composite hydrogen storagematerial is formed having a mesoporous metal-organic scaffoldingframework with a median pore size of about 2–4 nm, and a surface ofgreater than 500 m²/g. The mesoporous metal-organic scaffolding materialis coated with hydrogen storage material such as LiB₂N₂H₁₃, LiBNH₇,Li₂BNH₈, 0.2 mol LiAlH₄–0.8 mol BH₃NH₃ mixture and mixtures thereof.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A hydrogen storage structure comprising: a mesoporous scaffoldmaterial having an average pore diameter of about 1–5 nm and a surfacearea of greater than 450 m²/g; and a hydrogen storage compositioncomprising X—H, Y—H and A—H bonds, where X comprises a Group 13 element,Y comprises a Group 15 element and A comprises one or more elementsselected from the group consisting of Group 1, Group 2 and mixturesthereof.
 2. The hydrogen storage structure of claim 1, wherein Xincludes boron and Y includes nitrogen.
 3. The hydrogen storagestructure of claim 1, wherein said A—H is a metal hydride.
 4. Thehydrogen storage structure of claim 3, wherein said metal hydride islithium hydride (LiH).
 5. The hydrogen storage structure of claim 3,wherein said metal hydride is lithium aluminum hydride (LiAlH₄).
 6. Thehydrogen storage structure of claim 1, formed from precursor (a)comprising X—H and Y—H bonds and precursor (b) comprising said A—Hbonds; said (a) and (b) being in an atomic proportion to one anothersufficient to provide a composition expressed by the nominal generalformula A_(q)X_(r)Y_(s)H_(t) where the atomic ratio of q:r is greaterthan 0 and less than 3, the atomic ratio of s:r is greater than 0 andless than 2 and the atomic ration of t:r is greater than 0 and less than9.
 7. The hydrogen storage structure of claim 6, where A comprises Li, Xcomprises B, Y comprises N and the atomic ratio of Li:B is greater than0 and less than 3, the atomic ratio of N:B is greater than 0 and lessthan 2, and the atomic ratio of H:B is greater than 0 and less than 9.8. The hydrogen storage structure of claim 6, wherein the proportion issufficient to provide a composition expressed by the nominal generalformula Li₂BNH₈.
 9. The hydrogen storage structure of claim 6, whereinthe proportion is sufficient to provide a composition where Li is about2, B is about 1, N is about 1, and H is about
 8. 10. The hydrogenstorage structure of claim 1, formed by reacting said hydride with saidhydrogen storage composition comprising X—H and Y—H bonds to provide aresultant composition comprising A_(q)X_(r)Y_(s)H_(t), where q, r, s,and t are each greater than 0 and selected to provide electroneutrality.11. The hydrogen storage structure of claim 1, formed from precursor (a)comprising borazane and precursor (b) comprising hydride, each saidborazane and hydride in an amount sufficient to cause a greaterproportion of hydrogen in the system to be released in the form ofhydrogen gas and a lesser proportion of hydrogen in the system to bereleased as bound in other byproducts, as compared to the release ofhydrogen from borazane alone.
 12. The hydrogen storage structure ofclaim 11, where, on the basis of 100 moles of hydride and borazane, theamount of hydride is greater than 0 and less than
 100. 13. The hydrogenstorage structure of claim 11, wherein the hydride is lithium aluminumhydride.
 14. The hydrogen storage structure of claim 11, wherein thehydride is lithium hydride.
 15. The hydrogen storage structure of claim1, wherein A is selected from the group consisting of Li, Na, K, Mg, Ca,and mixtures thereof.
 16. The hydrogen storage structure of claim 1,wherein Y includes phosphates.
 17. The hydrogen storage structure ofclaim 1 wherein the scaffolding material is selected from the group ofsilica-based material, metal-organic framework material, zeolite typematerial, alumina-based material, carbon-based mesoporous material, andcombinations thereof.
 18. The hydrogen storage structure of claim 1wherein the mesoporous scaffold material has a median pore size of about2–4 nm.
 19. The hydrogen storage structure of claim 1 wherein themesoporous scaffold material has a surface area of greater than 500m²/g.
 20. A hydrogen storage structure comprising: a mesoporous scaffoldmaterial having an average pore diameter of about 1–5 nm and a surfacearea of greater than 450 m²/g; a hydrogen storage compositionrepresented by the nominal general formula A_(q)X_(r)Y_(s)H_(t): where Xcomprises a Group 13 element, Y comprises a Group 15 element and Acomprises one or more elements selected from the group consisting ofGroup 1, Group 2 and mixtures thereof; and where the atomic ratio of q:ris greater than 0 and less than 3, the atomic ratio of s:r is greaterthan 0 and less than 2 and the atomic ration of t:r is greater than 0and less than 9; and said composition comprising at least one phase thatis an a phase having two borazane molecules linked together through alithium bridge.
 21. The hydrogen storage composition of claim 20,wherein the α phase has the equivalent of more than two BH₃NH₃ moleculesper unit cell.
 22. The hydrogen storage composition of claim 20, whereinthe α phase is substantially a tetragonal crystal structure.
 23. Thehydrogen storage composition of claim 20, wherein the α phase is asubstantially tetragonal crystal structure with a P-42₁c space group.24. The hydrogen storage composition of claim 20, wherein the α phase isa substantially tetragonal crystal structure with lattice perimeter a ofabout 4.032 angstroms and lattice perimeter of about 17.001 angstroms.25. The hydrogen storage composition of claim 20, having a β phasedifferent from said α phase.
 26. The hydrogen storage composition ofclaim 25, having a γ phase different from said α phase and β phase. 27.The hydrogen storage composition of claim 26, wherein said α phase isnominally LiB₂N₂H₁₃, said β phase is nominally LiBNH₇, and said γ phaseis nominally Li₂BNH₈.
 28. The hydrogen storage composition of claim 20,with said α phase having an X-ray diffraction (XRD) patternsubstantially as shown as composition n=½ in FIG.
 1. 29. A hydrogenstorage material comprising: a mesoporous material having a porediameter of about 2–4 nm and a surface area of greater than 450 m²/g;and a hydrogen storage compound selected from the group comprising ofLiB₂N₂H₁₃, LiBNH₇, Li₂BNH₈, a mixture of 0.2 mol LiAlH₄ and 0.8 molBH₃NH₃, and mixtures thereof.
 30. A hydrogen storage materialcomprising: a mesoporous material having an average pore size of 2–4 nmselected from the group comprising silica-based mesoporous materials,carbon-based mesoporous materials, alumina based mesoporous materials,zeolite mesoporous materials, metal-organic framework mesoporousmaterials, and mixtures thereof; and a hydrogen storage composition X—H,Y—H and A—H bonds, where X comprises a Group 13 element, Y comprises aGroup 15 element and A comprises one or more elements selected from thegroup consisting of Group 1, Group 2 and mixtures thereof.
 31. Ahydrogen storage composite material comprising: a mesoporous scaffoldmaterial; and a hydrogen storage composition comprising X—H, Y—H and A—Hbonds, where X comprises a Group 13 element, Y comprises a Group 15element and A comprises one or more elements selected from the groupconsisting of Group 1, Group 2 and mixtures thereof, wherein the heat ofdecomposition is lower for the composite than the hydrogen storagecomposition.